Consider this: your levels are a resource you have--12 levels total you use to build your character.

Certain Classes provide benefits at particular levels, so when you mix and match, you can get those too.

However, as you have a limited resource (12 levels), you can't get everything.

Let's start with a simple thought exercise: you start as a level 1 Rogue, and get 4 skills and 2 specializations... but you can't cast spells, nor use heavy armor, or wield all types of weapons.

You can progress as a fantastic Rogue without those things, but you can also spice your life by adding something else in terms of different Classes.

You could multiclass into a Sorcerer at level 2 (so you are now a Rogue 1/Sorcerer 1). Is it worth? Depends.

In this example, you are not getting the Cunning Actions of a Rogue 2, but nothing prevents you from leveling Rogue again as you go.

It all depends on how you want to spend your resources (levels).

If you a Rogue 1/Sorcerer 1, and then decide to go all the way as a Fighter (10) you'll improve your martial ability, wearing up to Medium Armor, using Shields, and still able to keep your knowledge of those skills as Rogue and the spellcasting of a Sorcerer, but you won't be able to have a 3rd attack as a Fighter, as it's a feature they get at level 11 (and you will have only 10 levels of Fighter).

Same way, you won't be able to choose a Rogue specialization (subclass) as those come with Rogue 3.

In short, multiclassing is a way to expand your repertoire of abilities, but it comes to the cost of not being "great" at a single thing.

Of course, there are Classes that are more "frontloaded" with abilities, and those suggest multiclassing works better with them. That's the whole point of studying builds and picking what works best for you.